Castor is a temporal transcription factor that specifies early born central complex neuron identity

The generation of neuronal diversity is important for brain function, but how diversity is generated is incompletely understood. We used the development of the Drosophila central complex (CX) to address this question. The CX develops from eight bilateral Type 2 neuroblasts (T2NBs), which generate hundreds of different neuronal types. T2NBs express broad opposing temporal gradients of RNA-binding proteins. It remains unknown whether these protein gradients are sufficient to directly generate all known neuronal diversity, or whether there are temporal transcription factors (TTFs) with narrow expression windows that each specify a small subset of CX neuron identities. Multiple candidate TTFs have been identified, but their function remains uncharacterized. Here, we show that: (1) the adult E-PG neurons are born from early larval T2NBs; (2) the candidate TTF Castor is expressed transiently in early larval T2NBs when E-PG and P-EN neurons are born; and (3) Castor is required to specify early born E-PG and P-EN neuron identities. We conclude that Castor is a TTF in larval T2NB lineages that specifies multiple, early born CX neuron identities.